1. Technical Field
The present invention relates to the fields of mining and water reclamation, and more particularly to a process for removing colloid-forming metals from aqueous solutions containing those metals in ionic form.
2. Background of the Invention
Metal mining operations often result in an ecological waste land surrounding the mine. In addition to unsightly mounds of mine spoil that often dot the adjacent landscape, metal salts and oxides present in those spoil mounds as sulfides or are oxidized over time to sulfuric and sulfurous acids. Those acids interact with other salts present to form hydrochloric and nitric acids that further dissolve ores present in the spoil mounds, releasing metallic ions to the local aqueous environment and water table. In addition, similar conditions exist within the mines themselves, which leads to a build up within abandoned mines of water containing unacceptable amounts of dissolved metals, many of which are poisonous to plants, animals and man.
The states of Colorado, California, Alaska, Wyoming and Montana in the U.S.A. contain several abandoned mines containing metal ion-contaminated waters within the mines themselves or on nearby lands. Such water contamination can also occur in wells, springs, and both liquid and geothermal flows.
Aside from the toxic metals present in such waters, some waters also contain dissolved precious metals such as gold, silver and the platinum group metals including platinum, palladium, and rhodium, as well as copper. Some of these waters are relatively free of toxic metal ions, whereas other waters contain extremely toxic concentrations of poisonous metal ions. Less economically important metals that can also be present include iron, manganese, zinc, lead, tin, and arsenic.
Water containing colloid-forming metal ions that is present at mining operation sites, as in the mines themselves or draining from spoil mounds at mining sites, or water draining from off site spoil mounds, as well as streams and aquifers polluted by run off or seepage from mining operations is collectively referred to herein as mine waste water. It would be economically beneficial if those colloid-forming metals could be removed from the aqueous mine waste water solutions that contain them. Those economic benefits can come from recovery and sale of the metals themselves such as gold, silver and the platinum group, as well as from the removal of the metal ions present to provide less polluted, useful water. The process described below provides one such process that can remove colloid-forming metals from metal ion-containing water.
A process is contemplated for obtaining colloid-forming metal particles from an aqueous solution containing those metals in ionic form. In accordance with a contemplated process, an aqueous mine waste water solution containing a desired colloid-forming metal in ionic form is provided having a pH value of about 4.0 to about 6.5. Direct current at about 1.2 to about 2.0 volts is passed through the solution for a time period sufficient to convert the desired metal ions into colloidal metal and form a colloidal metal-containing composition. The solution is preferably agitated during that passage of current. Cellulose particles are admixed with agitation with the colloidal metal-containing composition to form an agitated slurry of cellulose particles admixed, preferably homogeneously admixed, with the colloidal metal. A flocculating agent is thoroughly admixed with the agitated slurry to form a settling composition containing a cellulosic floc. The settling composition is maintained in the absence of agitation for a time period sufficient to form two phases, a first phase that contains water and a solid phase comprising colloidal metal entrapped within a cellulosic floc and a second, aqueous, liquid phase having a reduced concentration of desired metal (xe2x80x9cdischarge waterxe2x80x9d). The solid and liquid phases are separated. The solid phase is dried, fluxed and heated to a temperature sufficient to melt the colloidal metal present to form larger particles of the desired metal and to burn off the cellulosic floc. The desired metal larger particles are typically collected. The separated liquid phase is usually returned to the source from which it was obtained, albeit typically at a down stream position relative to the position from which it was obtained.
A contemplated process can be carried out in a batch process, as a continual or a continuous process, as is desired. In preferred processes, the desired colloid-forming metal is gold or a platinum group metal.
The present invention has several benefits and advantages.
One benefit is that use of a contemplated process can result in the economic recovery of metals from mine waters having about 5 to about 1500 ppb of a desired metal such as gold.
An advantage of the present invention is that its use can provide enhanced water quality through removal of metal ions from mine waters.
Another benefit of the invention is that greater than about 90% of the desired metal present in the original mine water can be recovered using a contemplated process.
Still further benefits and advantages of the invention will become apparent to the skilled worker from the description that follows.